User`s manual

DIGITAL-LOGIC AG MSLB-P5 Manual V0.3
PRELIMINARY 40
4.3.7 Driver for COM3-COM4 RS422/485
In the case of RS422 /485 a driver must handle the RTS and DTR control signall, to prevent of buscol-
lisions and to become a proper bus access. This driver s must be programmed operating system de-
pendent and/or application dependent.
The modemcontrol register is defined as follow:
Adress COM3: 3Ech
Adress COM4: 2ECh
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
reserved Reserved reserved reserved OUT2 OUT1 RTS DTR
Examples:
To Enable Rx and Tx
Mov dx,3Ech ; Select register 3Ech for COM3
Mov al, 02 ; Enable Receiver and Transmitter
Out dx,al ; Set the value
To Disable Rx and Tx
Mov dx,3Ech ; Select register 3Ech for COM3
Mov al, 01 ; Enable Receiver and Transmitter
Out dx,al ; Set the value
To Enable Rx and Disable Tx (for RS485)
Mov dx,3Ech ; Select register 3Ech for COM3
Mov al, 00 ; Enable Receiver and Disable Transmitter
Out dx,al ; Set the value
For COM use the adress 2Ech.